The rigid adherence to National Dex limitations, specifically in gen 4.
for HGSS, there’s a decent amount of criticism for Gen 2 for the minutiae of Pokémon collection (waiting for swarms, specific event days, etc…). HGSS also introduces this badass idea of regional sounds, to bring in even more variety. Who wouldn’t love the opportunity to bring in another steel and dark type (Bronzor and Absol), Gen 2’s new additions that are still exceedingly rare? Tossing an iconic bird Chatot (sorry Hoothoot) in Sprout Tower to finally give Violet City, steeped in tradition, a tradition of bird Pokémon? Adding in some cool fighting types, which Gen 2 was definitely lacking in providing? Shinx in the National Park for anyone wanting to do an Officer Jenny run? For what little they were, radio sounds offered a ton of depth and it was already soft locked behind two gym badges in having to get the Radio Card. Buuuut no, you can only unlock this after the National Dex, after you’ve beaten the Elite Four and are taking on Kanto (which only exacerbates the issue of catching low level Pokemon and fighting strong battles).
Same story again for Platinum and the GBA cartridge offerings. The chance to start with a Solrock from the very first lake sounds awesome. However, this opportunity, which already requires the separate purchase of another game, is locked behind a National Dex.
There are just several opportunities in which GF/Nintendo can loosen regional restrictions to the tune of 10-15 Pokémon in the face of 493 or whatever that would add just so much depth, but they won’t.